Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
BALANCE at Dec. 31, 2013	$ 247,192	$ 4	$ 790,424		$ (543,236)
BALANCE (in shares) at Dec. 31, 2013		37,752,000
Exercise of options for common stock	$ 980		980
Exercise of options for common stock, shares	86,007	86,000
Exercise of warrants for common stock	$ 19,536		19,536
Exercise of warrants for common stock, shares		766,000
Vest of restricted stock units, shares		7,000
Shares withheld for taxes	(34)		(34)
Shares withheld for taxes, shares		(1,000)
Grant of restricted stock awards, net of cancellations, shares		6,000
Issuance of common stock for cash, net of offering costs	94,503		94,503
Issuance of common stock for cash, net of offering costs, shares		2,650,000
Issuance of common stock under employee stock purchase plan	1,015		1,015
Issuance of common stock under employee stock purchase plan, shares		46,000
Stock-based compensation	20,345		20,345
Unrealized loss from available-for-sale securities	(95)			$ (95)
Net loss	(135,789)				(135,789)
BALANCE at Dec. 31, 2014	247,653	$ 4	926,769	(95)	(679,025)
BALANCE (in shares) at Dec. 31, 2014		41,312,000
Exercise of options for common stock	$ 10,010		10,010
Exercise of options for common stock, shares	816,696	817,000
Shares withheld for taxes	$ (182)		(182)
Shares withheld for taxes, shares		(6,000)
Grant of restricted stock awards, net of cancellations, shares		181,000
Issuance of common stock for cash, net of offering costs	119,916	$ 1	119,915
Issuance of common stock for cash, net of offering costs, shares		3,250,000
Issuance of common stock under employee stock purchase plan	879		879
Issuance of common stock under employee stock purchase plan, shares		76,000
Stock-based compensation	32,117		32,117
Unrealized loss from available-for-sale securities	(16)			(16)
Net loss	(220,030)				(220,030)
BALANCE at Dec. 31, 2015	190,347	$ 5	1,089,508	(111)	(899,055)
BALANCE (in shares) at Dec. 31, 2015		45,630,000
Exercise of options for common stock	$ 19,353		19,353
Exercise of options for common stock, shares	1,056,821	1,113,000
Shares withheld for taxes	$ (2,168)		(2,168)
Shares withheld for taxes, shares		(47,000)
Grant of restricted stock awards, net of cancellations, shares		50,000
Issuance of common stock for cash, net of offering costs	364,749		364,749
Issuance of common stock for cash, net of offering costs, shares		7,876,000
Issuance of common stock under employee stock purchase plan	1,577		1,577
Issuance of common stock under employee stock purchase plan, shares		137,000
Stock-based compensation	30,107		30,107
Unrealized loss from available-for-sale securities	(9)			(9)
Net loss	(267,265)				(267,265)
BALANCE at Dec. 31, 2016	$ 336,691	$ 5	$ 1,503,126	$ (120)	$ (1,166,320)
BALANCE (in shares) at Dec. 31, 2016		54,759,000